Elemental sulfur frequently occurs in nature intimately admixed with various mineral impurities. Where such deposits occur at or near the surface, the traditional "Frasch" recovery method cannot be used. As a result, such deposits cannot be economically exploited.
Elemental sulfur may also be produced by certain chemical reactions, and in particular by the oxidation of the sulfide sulfur in metallic sulfides. Specific examples of such oxidation are the electrolytic oxidation of nickel sulfides as taught in U.S. Pat. No. 2,839,461, and the ferric chloride oxidation of copper sulfide as taught in U.S. Bureau of Mines Report of Investigation R.I. 7474.
Where elemental sulfur is produced by chemical reaction, its separation from mineral gangue is required in order that it be of sufficient purity to be a marketable by-product.
Thus, whether the combination of sulfur and mineral impurity occurs in nature or as a result of chemical reactions, the separation of the elemental sulfur and the mineral impurities has become important for economic reasons. Furthermore, there is increasing public concern over the large quantity of sulfur dioxide air contamination which results from the traditional smelting of metallic sulfides. Processes for the conversion of sulfides by the chemical or electro-chemical means cited above which produce elemental sulfur are of public benefit. The low cost and efficient separation of the elemental sulfur produced is an important adjunct to such processes.
In the past, various techniques have been proposed and used commercially to accomplish the separation of sulfur from mineral impurities. These have included melting and filtering, and sublimation. Neither process has proven to be economical in the presence of substantial quantities of mineral impurities. More recently, certain organic solvents have been proposed whereby the sulfur is dissolved and caused to reprecipitate in pure form. Such processes have been shown to be effective in processing low grade sulfur feeds and in producing pure products but are inherently expensive in capital and operating costs.
A process is taught in U.S. Pat. No. 2,537,842, McGauley et al., whereby an aqueous slurry of the sulfur is heated above the melting point of sulfur, and then cooled below the melting point, resulting in the formation of discrete sulfur particles which may be separated by froth flotation from the mineral gangue. However, in the patent there is no provision for removal of certain deleterious acid soluble metal ions associated with elemental sulfur produced from metallic sulfides. Thus, the elemental sulfur wets the mineral impurity surfaces and cannot be separated. Further, in the McGauley patent there is no provision for a prolonged heating time since the patent gives a maximum of 5 minutes and, finally, no provision for basic additives during that treatment.
Additionally, the process of U.S. Pat. No. 3,371,999, Skrzec, is of interest. In separating iron and sulfur chlorides contained in molten sulfur, the patentee passes the molten sulfur by-product countercurrent to a dilute basic aqueous solution to extract the chlorides and collect the elemental sulfur in a pool.
While peripherally relevant, substantial differences exist between the process of Skrzec and the present invention. In the patent there is no suggestion of a pre-wash to remove acid or water soluble deleterious ions. Further, there is no coalescence from mineral impurities and the amounts and nature of the alkaline treating agents or additives are different. Finally, the patent shows uniformly a short treating time while in the present process, optimum coalescence with respect to yield occurs after about 1 hour and optimally 2 hours (120 minutes).